Japanese citizens or any people of Japanese ancestry living in the United States were subject to internment after the outbreak of World War II. The pure and simple reason is that their patriotism was questioned, since U.S. entry into the war had been precipitated by the attach on Pearl Harbor, and it was known that Japanese spies in Honolulu had aided in the attack. One Japanese person in Hawaii had forwarded pictures of each of the ships in the Harbor, so the attacking planes knew exactly where each ship was docked. Sadly, all persons of Japanese ancestry were painted with the broad brush of possible espionage. Interestingly, no German Americans were interned, which raises the issue of whether race was a question in the internment. Race was a factor in anti-Japanese sentiment during the time period. One California barber shop hung a sign outside which read "free shaves for Japs. Not responsible for accidents."
The constitutionality of internment was challenged by a Japanese American citizen, but the Supreme Court upheld the government's right to do so in Korematsu vs. U.S.